Recently, intermetallic compounds, such as titanium-aluminum-based, nickel-aluminum-based, iron-aluminum-based and iron-cobalt-based compounds, have been attracting attention as heat-resistant materials.
Alloys in general have dissimilar atoms arranged irregularly at each lattice of the crystal structure. On the other hand, intermetallic compounds have regular structures with constituent atoms arranged at specific positions to exhibit interesting deformation behavior. For example, they can exhibit apparently abnormal phenomena in that their strength conversely increases as deformation temperature increases within a certain range. Such phenomena make these compounds noted as heat-resistant, high-strength materials.
Of these titanium-aluminum-based intermetallic compounds, TiAl and Ti.sub.3 Al are attracting attention as structural materials for engine members which are subject to high temperatures, such as those for aerospace devices and automobiles, because of their low density, lightness and high specific strength relative to heat resistance.
Titanium-aluminum-based alloys are generally produced by a melting method, in which stocks are molten and cast. This method has its problems, for example, component segregation (gravity segregation) in which titanium is separated from aluminum of lower density during the solidification step, and coarse grains which grow to 100 .mu.m or more and deteriorate the desired quality.
Moreover, the melting method requires an expensive melting unit, e.g., a vacuum arc melting furnace, and makes it difficult to produce titanium-aluminum-based intermetallic compounds having target characteristics.
A powder metallurgical process is considered an effective substitute for the above-referenced melting method. To produce titanium-aluminum-based intermetallic compounds in a powder metallurgical method, it is necessary to alloy powdered titanium and aluminum by mechanical milling in a pot (mill) with hard mixing balls. This includes repeated mixing, milling and adhesion under pressure of these components, in order to secure fine, isometric grains of uniform size and thereby to improve their mechanical properties. This process is referred to as mechanical alloying.
To produce titanium-aluminum-based metallic compound powders, mechanical alloying normally has been effected by ball-milling the stock powders of titanium and aluminum in an inert atmosphere (e.g., argon) or under a vacuum to prevent oxidation of the intermetallic compound powders.
However, this method is also subject to problems, for instance, low product yield can be provided even when the stock powders are milled for extended periods. This is because most powders are deposited on the balls and inner mill walls and are left unreacted. Although mechanical alloying is an effective means to produce a fine intermetallic compound powder of uniform particle size, as discussed above, such a low product yield is an economically critical disadvantage.
Attempts to prevent build-up of the powders in the inner mill walls include milling in a nitrogen or ammonia atmosphere, or in the presence of an organic solvent such as heptane. This method can increase synthesis yield to almost 100%.
Nevertheless, these methods are undesirable, because of formation by these additives of, for instance, carbides or nitrides, which may severely damage sinter structures and mechanical properties of the titanium-aluminum-based intermetallic compound powder. Therefore, the problems involved in the mechanical alloying of titanium and aluminum powders have not been drastically solved.
In an attempt to solve these problems, the use of hydrogenated titanium powder has been proposed in place of the use of pure titanium powder. This method can provide a powder structure with fine hydrogenated titanium and .alpha.-titanium particles dispersed in aluminum, while increasing synthesis yield to 95% or more without forming a carbide or nitride. A titanium-aluminum-based metallic compound sinter can be readily obtained, because hydrogen is dissociated at around 500.degree. C. Furthermore, hydrogenated titanium powder is 20 to 30% less expensive than pure titanium powder for the conventional mechanical alloying process, which is yet another advantage of this method.
However, the use of fine, pure titanium powder as the suitable stock for mechanical alloying is very expensive. Even when hydrogenated titanium powder is utilized because it is 20 to 30% less expensive than pure titanium powder, the conventional mechanical alloying process still lacks commercial viability.